The influences of different fibers and F ions on the formation mechanism of hierarchical structure were investigated. This study was undertaken to better understand how fibers affect the growth of mullite whisker and how the fluoride influences the transition of new phases. In this work, three networks with different fibers and the same active powder were prepared to investigate the effect of fibers on the mullite nucleation. The scanning electron microscope, X-ray diffraction and thermal gravity analysis and differential scanning calorimetry were employed to investigate the morphology, phase and weight changes during the fabrication process. The results showed that mullite fibers did not only decrease the reaction temperature but also promote mullite whiskers to grow on the fiber surface preferentially. Moreover, it was also certified that topaz was not the necessary intermediate to form mullite whisker. The investigation of AlF3 illustrated it departed at about 600°C largely lower than the evaporation temperature of AlF3 to react with Al and Si.